Recently, capacitive electromechanical transducers produced by a micromachining process have been actively studied. A typical capacitive electromechanical transducer includes a lower electrode, a vibrating membrane supported above the lower electrode with a certain distance therebetween, and an upper electrode provided on the surface of the vibrating membrane. Such a transducer is used as, for example, a capacitive micromachined ultrasonic transducer (CMUT), which is one form of a capacitive electromechanical transducer.
Such a capacitive micromachined ultrasonic transducer includes a lightweight vibrating membrane, which is vibrated by applying an electric field of a certain frequency between the upper electrode and the lower electrode. Thus, the capacitive micromachined ultrasonic transducer can transmit ultrasonic waves. On the other hand, when the vibrating membrane is vibrated by ultrasonic waves, the transducer receives the ultrasonic waves by detecting a change in the capacitance between the upper electrode and the lower electrode, and the ultrasonic waves can be taken out as electrical signals. Such capacitive micromachined ultrasonic transducers that exhibit good broadband characteristics even in liquids and air can be easily obtained. If such CMUTs are applied to, for example, the medical field, medical diagnosis with an accuracy higher than that of existing medical diagnosis can be realized, and thus the CMUTs have attracted attention as a promising technology.
Next, the principle of operation of a capacitive electromechanical transducer will be described. In transmitting elastic waves (typically ultrasonic waves), an alternating current (AC) potential (voltage) superimposed on a direct current (DC) potential (voltage) is applied between a lower electrode which is a first electrode and an upper electrode which is a second electrode. By applying an electric field between the first electrode and the second electrode in this manner, the vibrating membrane is vibrated by an electrostatic force that acts between the first electrode and the second electrode to generate elastic waves including ultrasonic waves. On the other hand, in receiving ultrasonic waves, since the vibrating membrane is deformed by the ultrasonic waves, signals are detected by a change in the capacitance between the lower electrode and the upper electrode, the change being caused by the deformation. Mechanical energy and electrical energy can be converted from one to the other by the principle described above. The theoretical sensitivity of such a capacitive electromechanical transducer is inversely proportional to the square of the distance (also referred to as “gap”) between the electrodes. In order to manufacture a transducer with high sensitivity, the gap is controlled to be 100 nm or less.
In a typical method for forming a gap of a capacitive electromechanical transducer, a sacrificial layer having a thickness equal to a desired distance between electrodes is formed, a vibrating membrane is formed on the sacrificial layer, and the sacrificial layer is then removed. Such a technique is disclosed in PTL 1 and NPL 1.